1. Field of the Invention
Embodiments of the present invention relate to aircraft control systems and methods. More particularly, embodiments of the present invention relate to a method and apparatus for estimating future pitch and roll angles of an aircraft.
2. Description of the Related Art
The attitude (or angular orientation) of a vehicle encompasses both its pitch angle and roll angle (or bank angle). For an aircraft, the pitch angle generally refers to the nose-up/nose-down angle, and the roll angle generally refers to the angle of the wings (e.g. left wing up/right wing down or left wing down/right wing up; similar concepts for helicopters). Aircraft autopilot systems use attitude information to control an airplane's angular orientation, altitude, airspeed, vertical speed, and direction of flight.
Aircraft avionics systems receive information from a variety of flight instrumentation systems, including digital attitude and heading reference systems (AHRS) and digital air data computers (ADC). For some applications, particularly for retrofit applications into already-fielded airplanes, the AHRS and/or ADC may be interfaced with an autopilot system having an existing certification basis that presumes an interface with a different type of attitude sensor, such as an older analog attitude sensor or another model of a digital AHRS. This can be problematic when the AHRS and/or ADC being interfaced with the autopilot system have different dynamics, including latency or time lag properties, than the original analog attitude sensor or the other model of the digital AHRS.
For example, many AHRSs have greater time lag or time delay than some analog vertical gyros that are being replaced. In one application, the AHRS has an effective overall time delay of 40 msec to 50 msec over a typical frequency range between 0 Hz and 2 Hz. The time delay from the AHRS may result in undesirable autopilot performance if the associated vertical gyro is replaced by the AHRS. Since the AHRS constitutes part of the autopilot system's control loop, the performance of the autopilot system may depend upon the effective time delay introduced in the pitch and roll values by the AHRS.
One option to overcome the dynamics between what the autopilot system has been designed for and what the AHRS outputs is to alter the autopilot system. However, altering the autopilot system adds to the cost and labor required to complete the retrofit.